Under certain operating conditions, engine cylinders can misfire or have slow burns. These misfires or slow burns can increase hot residuals entering an adjacent cylinder during cam or valve overlap periods. When this happens, the hot residuals can cause the misfire to pre-ignite within the adjacent cylinder. The pre-ignition can generate very high in-cylinder pressures that can result in combustion pressure waves similar to combustion knock, but with larger intensity.
Also, in boosted engines, late burn combustion events wherein the combustion is later than intended can also lead to pre-ignition combustion events. Specifically, the late combustion can lead to high exhaust manifold pressures and temperatures, as well as higher than intended exhaust residuals. The elevated exhaust manifold pressures can overcome the exhaust valve spring pressure and potentially open exhaust valves on adjacent cylinders, filling the cylinder with hot exhaust gas residual and increasing the likelihood of pre-ignition in the adjacent cylinders. The issue may be exacerbated in small volume exhaust manifolds, such as may be used in turbocharged engine systems to minimize the time to torque. As such, pre-ignition events can reduce engine performance and cause engine degradation.
Various approaches have been developed to reduce such late-burn induced pre-ignition events. One example approach is shown by Glugla et al. in US20150167573. Therein, in response to a misfire event, or a late burn event in a first cylinder, a second neighboring cylinder that is most likely to receive the hot residuals from the first cylinder is determined. One or more pre-ignition mitigating actions are then preemptively performed in the second cylinder before pre-ignition is induced therein.
However the inventors herein have identified potential issues with such an approach. As an example, based on the actual location, magnitude, and timing of exhaust pressure peaks during the late burn, an identity of the cylinder getting the hot residuals may differ. For example, the approach of Glugla may rely on an average exhaust pressure to identify the cylinder receiving the residuals. However, due to the exhaust pressure pulsations, the instantaneous value of the exhaust pressure may be significantly different from the average pressure, resulting in errors in identifying the residual receiving cylinder. Still other variables such as engine speed, spark timing, and valve timing events can affect the transport delay incurred between the generation and release of the hot residuals from a first cylinder and unintended receipt of the hot residuals in a second cylinder. As another example, the approach of Glugla may apply a more aggressive pre-ignition mitigating action (such as higher degree of richness of a pre-ignition mitigating enrichment, or a longer duration of enrichment) on the second cylinder than is warranted based on the amount of residuals actually received in the cylinder. As a result, fuel economy may be degraded.
The inventors herein have recognized that by determining an exhaust pressure profile for each cylinder by sampling cylinder exhaust pressure during an exhaust valve event of the cylinder, it may be possible to more accurately identify when and where residuals are being released, how much and how hot the residuals are, and which cylinders are going to be affected by those residuals. In one example, the above issues may be at least partly addressed by a method for addressing pre-ignition in an engine comprising: indicating imminent pre-ignition in a cylinder responsive to an amount of residuals received or generated in the cylinder, the amount of residuals based on exhaust pressure and intake manifold pressure; and adjusting fueling of the cylinder based on the indication and further based on the amount of residuals. In this way, misfire and late burn induced pre-ignition events may be reduced.
As an example, an exhaust pressure profile may be determined for each cylinder by sampling an exhaust pressure (estimated at an exhaust port) of each cylinder during corresponding exhaust valve events. The profile may include instantaneous exhaust pressure estimates for each point over the exhaust valve event, as well as a peak exhaust pressure, and an average exhaust pressure. Based on the exhaust pressure profile of each cylinder, an engine controller may determine whether a cylinder misfire event or a late burn event has occurred. In addition, the controller may determine an amount, location, and temperature of residuals released from the misfiring/late-burning cylinder by comparing the exhaust pressure to the intake manifold pressure. Based on the engine configuration, cylinder firing order, and valve timing of each cylinder, a cylinder that is likely to receive the residuals may be identified. Then, based on the amount and temperature of residuals released, the controller may further confirm whether the residuals will be received in the identified cylinder (such as by comparing the force generated by the residuals to the force of a spring holding down a valve seat). If residuals are received, responsive to the temperature and amount of the received residuals, a pre-ignition mitigating action to be performed in the identified cylinder may be adjusted. In addition, an internal EGR estimate in the residual releasing and receiving cylinders may be updated, allowing for cylinder operating parameters such as spark timing and external EGR to be correspondingly adjusted.
In this way, by estimating exhaust pressure pulsations in each engine cylinder, an amount of exhaust residuals released from a cylinder, and received in another cylinder, may be more accurately determined. The technical effect of relying on instantaneous and peak exhaust pressure estimates over a cylinder exhaust valve event, rather than an average exhaust pressure, is that a higher signal to noise ratio may be provided, improving the identification of misfiring or late-burning cylinders, as well as other cylinders affected by the residuals released from the misfiring or late-burning cylinders. By performing a pre-ignition mitigating adjustment in the cylinder identified as receiving the residuals, late-burn/misfire induced pre-ignition may be better addressed. The technical effect of adjusting fuel and/or spark in the affected cylinder based on the amount and temperature of residuals received therein is that pre-ignition may be addressed without degrading fuel economy. In particular, the fuel and spark adjustment may be more accurately matched to the degree of pre-ignition anticipated based on the residuals. Further still, by relying on the exhaust pressure profile for misfire detection, there may be fewer false positive indications of misfire. Overall, engine degradation due to misfire and late-burn induced pre-ignition can be reduced.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.